In addition to what others mentioned above about the threads possibly getting deleted, the discretion is up to the moderator and could be because of

1) duplicate/redundant posting. If you basically said the same thing in a different way a previous poster wrote, then it might not add value
2) too short of a post/incomplete post
3) if the mod felt it was spam
4) wrong categorization, which which case they often just move it to the correct category
5) inappropriate behavior/posting
6) off topic or no relevancy to the original question/thread topic.

Hope that helps.